Lean NOx Capture and Reduction by NH3 via NO+ Intermediates over H-CHA at Room Temperature

The oxidation of NO to NO 2 and the subsequent reduction by NH 3 via a NO + intermediate over a proton-type chabazite zeolite (H-CHA) were investigated by the combination of in situ infrared (IR) spectroscopy and density functional theory (DFT) calculations. The in situ IR spectral results indicate that the NO + species formed under a flow of NO + O 2 at 27–250 °C are more stable at lower temperatures over both H-CHA and copper-cation-exchanged CHA zeolite (Cu-CHA). The Arrhenius plot ( T = 27–120 °C) shows a negative apparent activation barrier energy (−11.5 kJ mol –1 ) for the formation of NO + species under the NO + O 2 flow over H-CHA. The time course of the IR spectra at 27 °C shows that NO is oxidized by O 2 to NO 2 and then further converted via N 2 O 4 to NO + and NO 3 – . The subsequent exposure to NH 3 at 27 °C reduces the NO + species to N 2 . DFT calculations revealed that Brønsted acid sites in zeolite pores promote the dissociation of N 2 O 4 intermediates into NO + and NO 3 – species with a low activation barrier (15 kJ mol –1 ). Moreover, the computed activation barrier for the reduction of NO + species by NH 3 was considerably low (6 kJ mol –1 ). The experimental and theoretical results of this study demonstrate the high potential of Cu-free H-CHA zeolites for promoting lean NO x capture to form NO + species and the subsequent reduction by NH 3 at room temperature.